Which is a factor on determining the average atomic mass of an element

Relative abundance of each isotopes of an element.

Explanation:

Consider the following example:

Uranium is used in nuclear reactors and is a rare element on earth. Uranium has three common isotopes. If the abundance of 234U is 0.01% the abundance of 235U is 0.71% and the abundance of 238U is 99.28% what is the average atomic mass of uranium

Abundance of U²³⁴ = 0.01%

Abundance of U²³⁵ = 0.17%

Abundance of U²³⁸ = 99.28%

Average atomic mass = ?

Solution:

Average atomic mass of uranium = (abundance of 1st isotope * its atomic mass) + (abundance of 2nd isotope * its atomic mass) + (abundance of 3rd isotope * its atomic mass) / 100

Average atomic mass of uranium = (234*0.01) + (235*0.71) + (238*99.28) / 100

Average atomic mass of uranium = 2.34 + 166.85 + 23628.64 / 100

Average atomic mass of uranium = 23797.83 / 100

Average atomic mass of uranium = 237.98 amu.